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Magnetic switch ? for the electrically knowledgeable

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Forum topic by AandCstyle posted 05-27-2016 03:32 PM 644 views 0 times favorited 12 replies Add to Favorites Watch
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AandCstyle

2575 posts in 1724 days


05-27-2016 03:32 PM

Topic tags/keywords: question

I couple weeks ago I mentioned that I seemed to be having a lot of electrical issues. I briefly mentioned my table saw while asking about my sander. The table saw finally completely died and I found the out that the switch was the source of the issue. I replaced the switch and the TS ran fine for the test run, but my question is why it shorted out in the first place? At least, it looks like a short to me. Whatever the cause, I would like to know so I can avoid a recurrence. Thanks!

-- Art


12 replies so far

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Loren

8314 posts in 3115 days


#1 posted 05-27-2016 03:45 PM

The copper contacts on mag switches get corroded over
time with use. To some extent the can be cleaned but
eventually they get mis-shaped and don’t make a good
contact, so they chatter on start up and eventually cook
the switch.

In a way that’s the idea I think. The switch gets smoked,
not the rest of the electrics. On more specialized industrial
machines the wiring can be pretty sophisticated for code
reasons and very much a PITA to repair if damaged.

I smoked a worn magnetic switch on a 7.5hp wide belt
sander by starting the machine using a 5hp phase converter.
Lesson learned. I upgraded to a bigger phase converter
and replaced the magnetic contactor. Switches and
contactors can be got pretty reasonably on ebay, shipped
from Asia. The tricky part is figuring out what current
Asian model is right to replace a 30 year old one.

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splintergroup

829 posts in 689 days


#2 posted 05-27-2016 04:38 PM

Art, it’s hard to fully diagnose over a single photo, but based on the usual mode for these kind of failures Loren is correct.

The damage in the photo looks to be heat related in the sense that a dead short somewhere would trip a protective breaker and not damage the wiring as bad. High resistance from a bad contact will cause things to heat up significantly, but not trip any protective devices. Your melty mess probably means the contacts got hot enough to start the ball rolling. This is a good example of why it is imperative to have a properly grounded tool.

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MrUnix

4244 posts in 1666 days


#3 posted 05-27-2016 05:19 PM

The damage in the photo looks to be heat related [...] High resistance from a bad contact will cause things to heat up significantly, but not trip any protective devices.
- splintergroup

That is exactly what it looks like. Typically caused by a loose or not fully secured wire or one that was undersized for the application. They can also get loose over time simply due to the thermal expansion/contraction of the wire each time the machine is used (and one reason copper wire is typically used as opposed to something like aluminium, which has a much higher thermal expansion rate).

Cheers,
Brad

-- Brad in FL - To be old and wise, you must first be young and stupid

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muleskinner

881 posts in 1903 days


#4 posted 05-27-2016 05:34 PM

Dirty, corroded or loose contacts cause resistance and arcing which end up with what you got there. A little fine wood dust and a little moisture are probably pretty common culprits in a woodshop. A guy should probably blow out the starters on his equipment every once in a while. And I should probably make a note to myself to do that sometime.

-- Visualize whirled peas

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Grumpymike

1918 posts in 1782 days


#5 posted 05-27-2016 09:02 PM

Muleskinner Hit it on the head resistance ... Each time the switch is turned off or on there is a small arc, leaving behind a small bit of carbon. Carbon is a resistor. A resistor in a circuit causes heat. More carbon (and other gunk) equals more heat until … China Syndrome and melt down.
How to avoid a switch failure? Just don’t use it. Like any electro-mechanical part there is failure … like a bearing wearing out.

-- Grumpy old guy, and lookin' good Doin' it. ... Surprise Az.

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clin

514 posts in 463 days


#6 posted 05-27-2016 09:25 PM



Muleskinner Hit it on the head resistance ... Each time the switch is turned off or on there is a small arc, leaving behind a small bit of carbon. Carbon is a resistor. A resistor in a circuit causes heat. More carbon (and other gunk) equals more heat until … China Syndrome and melt down.
How to avoid a switch failure? Just don t use it. Like any electro-mechanical part there is failure … like a bearing wearing out.

- Grumpymike

Some contact types actually rely on that small arc to keep the contact clean. But as all are saying, contacts do wear out and high resistance means heat.

Another possible issue is the connection of the wire to the switch. If this loosened up, you can also get a high resistance and heat. No different than a poor connection in house wiring.

Probably not a bad idea for any of us to periodically check those power connections in large machines, like table saws, and certainly if something is acting up.

-- Clin

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MadMark

978 posts in 920 days


#7 posted 05-27-2016 10:08 PM

The terminal crimp wasn’t correct …
M

-- Madmark - Madmark2150@yahoo.com Wiretreefarm.com

View AandCstyle's profile

AandCstyle

2575 posts in 1724 days


#8 posted 05-27-2016 11:43 PM

Thanks for the replies. My take away is that the switch is a part that is expected to wear out and has to be replaced periodically. That is fine, I just wanted to be certain that it wasn’t due to something that I am improperly doing.

Again, thank you!

-- Art

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runswithscissors

2192 posts in 1492 days


#9 posted 05-28-2016 04:48 AM

I could be flat out wrong about this, but I thought carbon was a conductor, not a resistor. Like the carbon brushes in a universal motor.

Not that that has any bearing on your switch problem.

-- I admit to being an adrenaline junky; fortunately, I'm very easily frightened

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TheFridge

5765 posts in 953 days


#10 posted 05-28-2016 05:11 AM

Pull it apart. It may be salvageable believe it or not. As long as the plastic support structure isn’t deformed or the contact pad isn’t burnt through. If it’s not salvageable just find you another IEC contactor with the same specs.

-- Shooting down the walls of heartache. Bang bang. I am. The warrior.

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Rentvent

148 posts in 316 days


#11 posted 05-28-2016 05:53 PM

I agree with MadMark – it looks like the crimp was bad.

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AandCstyle

2575 posts in 1724 days


#12 posted 05-28-2016 11:48 PM

Thanks for the responses.

-- Art

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